The endocervical canal is lined by simple columnar epithelium that secretes cervical mucus in a hormonally responsive manner. Estrogen stimulates production of a copious quantity of thin, watery, slightly alkaline mucus that is an ideal environment for sperm. Progesterone stimulates production of a scant, viscous, slightly acidic mucus that is hostile to sperm. During the normal menstrual cycle the conditions of the cervical mucus are ideal for sperm penetration and viability at the time of ovulation.
The Vagina
The vagina is one of the copulatory structures in women and acts as the birth canal (see
Figs. 44.13 44.25 ). Its mucosa is lined by a nonkeratinized stratified squamous epithelium. The mucosa has a thick lamina propria enriched with elastic fibers and is well vascularized. There are no glands in the vagina, so lubrication during intercourse comes from (1) cervical mucus (especially with intercourse that occurs midcycle), (2) a transudate (i.e., ultrafiltrate) from the blood vessels of the lamina propria, and (3) the vestibular glands. The mucosa is surrounded by a relatively thin (i.e., relative to the uterus and cervix) two-layered muscularis and an outer connective tissue. The vaginal wall is innervated by branches of the pudendal nerve, which contribute to sexual pleasure and orgasm during intercourse.
Hormonal Regulation During the Menstrual Cycle
The superficial cells of the vaginal epithelium are continually desquamating, and the nature of these cells is influenced by the hormonal environment. Estrogen stimulates proliferation of the vaginal epithelium and increases its glycogen content (referred to as “cornification”—but in humans, true cornification or keratinization does not occur). The glycogen is metabolized to lactic acid by commensal lactobacilli, thereby maintaining an acidic environment. This inhibits infection by noncommensal bacteria and fungi. Progesterone increases the desquamation of epithelial cells.
The External Genitalia
The female external genitalia are surrounded by the labia majora (homologues of the scrotum) laterally and the mons pubis anteriorly (see
Fig. 44.25 ). The vulva collectively refers to an area that includes the labia majora and mons pubis plus the labia minora, the clitoris, the vestibule of the vagina, the vestibular bulbs (glands), and the external urethral orifice. The vulva is also referred to as the pudendum by clinicians. The structures of the vulva serve the functions of sexual arousal and climax, directing the flow of urine, and partially covering the opening of the vagina, thereby inhibiting entry of pathogens.
The clitoris is the embryological homologue of the penis and is composed of two corpora cavernosa, which attach the clitoris to the ischiopubic rami, and a glans. These structures are composed of erectile tissue and undergo the process of erection in essentially the same manner as the penis. Unlike the penis, clitoral tissue is completely separate from the urethra. Thus the clitoris is involved in sexual arousal and climax at orgasm. The vagina is likewise involved in sexual satisfaction but also serves as the copulatory organ and birth canal.
Hormonal Regulation During the Menstrual Cycle
The structures of the vulva do not show marked changes during the menstrual cycle. However, the health and function of these structures are dependent on hormonal support. The external genitalia and vagina are responsive to androgens (testosterone and dihydrotestosterone) and estrogen. Androgens also act on the central nervous system (CNS) to increase libido in women.
Biology of Estradiol-17β and Progesterone
Biological Effects of Estrogen and Progesterone